1. Field of the Invention
This invention relates to a resin composition for masterbatch used for coloring black molded articles, such as a light-shielding film, a container for a light-shielding cartridge, a light-shielding spool, a light-shielding cassette and a light-shielding magazine.
2. Description of the Prior Art
In general, the thermoplastic resins used for coloring molded articles for photosensitive materials are colored by directly adding a coloring material such as pigment or dye to the thermoplastic resin as it is or by preparing a resin containing a coloring material in the form of pellets, flakes, beads, granules or the like. As the former method, there are the dry color method using a coloring material in solid state at ordinary temperature, the liquid color method using a coloring material in liquid state at ordinary temperature and the paste color method. While, as the later method, there are the masterbatch method (concentrate method) using the pellets containing a coloring material in a high concentration and the compound method which involves blending the total amount of a coloring material with a base resin, melting, kneading and pelletizing the resin composition in a kneading granulating machine, and molding colored resin articles using the pellets. The respective coloring methods have advantages and disadvantages, and utilize respective characteristics. Among them, the masterbatch method is superior is handling, no scattering upon a use, cost and the like. The applicant has developed a resin composition for masterbatch (Japanese Patent KOKAI No. 63-186740).
Incidentally, the blending ratio of a masterbatch resin to a thermoplastic resin used as the base resin for molded articles is reduced in order to prevent the change in the properties or to reduce cost, in molding various articles through various processes such as inflation process, T die extrusion, blow molding and injection molding. The resin composition for masterbatch must color the molded articles uniformly even in such a case. However, in the case of conventional resin compositions for masterbatch, when the light-shielding material concentration of the masterbatch resin is increased, the dispersion of the light-shielding material is inferior and agglomelation is liable to occur. Thus, the molded articles are not colored uniformly.
Moreover, film-molding time and injection molding cycle are shortened, and molded articles are rendered thinner. As a result, the masterbatch resin is required to disperse in base resins uniformly in a short time even at a low molding temperature where the degradation of the base resin is low. Therefore, it is necessary to improve the fluidity of the masterbatch resin at the molding temperature of the base resin. However, the fluidity of the masterbatch resin decreases with the increase of the carbon black concentration, and the masterbatch resin does not disperse in the base resin completely. As a result, various problems occur, such as irregular color of molded articles, fogging of photosensitive materials caused by insufficient light-shielding of the package, deviation of the color of the molded articles by the variation in the temperature and pressure of the molding machine, and the like. The above tendency is especially pronounced, in the case that the fluidity of the base resin is high.